The current technology that is used in underground mining transformer coil design is primarily conventional open wound and epoxy vacuum cast. Cooling transformers during operation has been a long standing problem and a variety of solutions have been used including use of coolant fluids, cooling tanks surrounding the transformer; transformers submerged in transformer oil; and the use of spacers in dry-type transformers that separate sections or layers of the transformer. A primary difference between the type of transformer and thus transformer cooling used, is the environment in which the transformer is used.
The typical operating environment of an underground mining transformer is such that it subjects the transformer to environmental contaminants such as moisture and coal dust which reduces the dielectric performance of the transformer over time. The expected load duty cycle is also very severe in underground mining environment subjecting the mine duty transformer to extreme swings in thermal loading as well as frequent short circuits. Another environmental challenge in the underground mining environment is real estate, resulting in reduced spacing between adjacent transformer coils which increases the environmental temperature surrounding the transformers.
The current industry accepted transformer construction is dry type (not oil filled or gas filled) non cast open wound construction that uses duct sticks to create air cooling ducts. This construction has always been problematic for the expected operating environment because it does not adequately prevent coil distortion when subjected to the mechanical stresses of a short circuit.
During a short circuit event, the magnetic forces tend to cause the primary winding to repel the secondary winding resulting in coil distortion. The coil tends to change shape such that the primary winding is forced outward causing the overall outside diameter of the coil to increase in physical size. This distortion of adjacent transformer coils combined with the reduced spacing between adjacent transformer coils can cause phase-to-phase failure. This phase-to-phase failure is extremely undesirable since it tends to decrease the electrical clearance between adjacent phases and can result in a phase-to-phase fault. The phase-to-phase failure also changes the impedance of the transformer. If the forces are large enough, the windings can also be forced out of the ends of the coils toward the core and clamping structure resulting in a phase to ground failure.
Another issue that frequently results from coil distortion is that the duct sticks used to create air ventilation passages will often loosen and fall out of the bottom of the coil reducing the air flow and efficiency of the cooling ducts. Thus, cooling ducts created using ducts sticks fail to solve the problems associated with reduced transformer spacing and harsh operating environments.
What is needed in this environment is a transformer with environmentally sealed transformer coils that incorporates rigid cooling ducts that are capable of withstanding extreme mechanical stresses.